1. Field of the Invention
The present invention relates to an information reproduction apparatus for reproducing information recorded on a recording medium, and, more specifically, to an Automatic Gain Control [AGC] circuit for controlling the amplitude of a reproduction signal read out from a recording medium at a constant level.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating an example of an AGC circuit for use in an information reproduction circuit of an optical disk. As is known well, fluctuations occur in a reproduction signal reproduced from a recording medium due to various factors. One known technique for avoiding this problem is to employ an AGC circuit which controls the gain of an amplifier for amplifying a reproduction signal so that the amplitude of the reproduced signal is maintained at a constant level. Two information recording techniques are known. One is a mark position recording technique, and the other is a mark edge recording technique. In the following description, an AGC circuit for use in conjunction with the mark position technique will be described. In the AGC circuit of FIG. 1, a bottom clamping circuit 2 is connected to the output of an AGC amplifier 1 so that negative portions of the reproduction signal amplified by the AGC amplifier 1 may be clamped by the bottom clamping circuit 2. The signal amplitude of the output signal of the bottom clamping circuit 2, that is, the amplitude of positive portions of the reproduction signal, is compared to a predetermined reference voltage Vref by an operational amplifier 3. The AGC amplifier 1 outputs a control signal corresponding to the difference obtained from this comparison. The gain of the AGC amplifier 1 is adjusted according to the central signal so that the amplitude of the reproduction signal will be at a level equal to the reference voltage. The reproduction signal, whose amplitude is maintained constant in the above-described manner, is subjected to signal processing, such as binarization, in a processing circuit of the following stage so as to generate reproduction data.
FIG. 2 is a block diagram illustrating another example of a conventional AGC circuit. In this AGC circuit, a reproduction signal is differentiated by a differentiator 4, whereby its amplitude is smoothed so as to remain at a constant level. Then the differentiated signal is input to an AGC amplifier 1. Full-wave rectification is performed on the output of the AGC amplifier 1 by a full-wave rectifier 5. The resultant output signal of the full-wave rectifier 5 is provided to an operational amplifier 3. The operational amplifier compares the full-wave-rectified signal provided by the full-wave rectifier 5 with a reference voltage Vref, and outputs a control signal corresponding to a difference between them. The gain of the AGC amplifier 1 is adjusted according to the control signal so as to maintain the amplitude of the reproduction signal at a constant level. In this case, a differentiated reproduction signal is output as a constant-amplitude reproduction signal.
The AGC circuit shown in FIG. 1, since the amplitude of the reproduction signal is directly detected, attenuation of high frequency components of the reproduction signal occurs due to modulation transfer function (MTF) characteristics of an optical system. As a result, the amplitude of the reproduction signal varies depending on the frequency. In a technique commonly used to avoid this problem, the signal level of a VFO signal disposed at the beginning of data of each sector of a recording medium is detected, and the gain of the AGC amplifier is determined based on the result of the detection, wherein the gain is fixed to this determined value during the signal reproduction of the other portion of the data. In such a technique, however, since the gain of the amplifier is fixed, if there is a fluctuation in reflectivity of a disk during reading of data in one sector or a fluctuation in reproduction laser power, then the amplitude of the reproduction signal varies depending on these fluctuations. Therefore, there is still difficulty with control to maintain the amplitude of the reproduction signal at a constant level.
Furthermore, a timing signal for representing a position of the VFO signal is required, which gives rise to an increase in the complexity of sequential control for starting an apparatus.
In the AGC circuit shown in FIG. 2, since the output signal is provided in the form of a differentiated signal, it has a problem that binarization of the reproduction signal cannot be performed correctly when it is used to reproduce a recorded signal according to the mark edge recording technique, while it can be used with no problems for mark position recording. In the mark position recording technique, a reproduction signal is converted into a binary signal in such a manner that the reproduction signal is differentiated, and then a zero-crossing point is detected so as to employ the location of this zero-crossing point as the location of a mark position. Thus, there is no problem when using the mark position recording technique.
On the other hand, in the reproduction of a signal recorded by the mark edge recording technique, the reproduction signal is compared with a slice level so as to detect the location of the mark edge. However, the AGC circuit shown in FIG. 2 can provide only a differential signal. One technique to obtain a binary mark edge signal from such a differential signal is to further differentiate the above differential signal to generate a second-order differential signal, whose zero-crossing location is detected so as to obtain a binary mark edge signal. However, in this technique, when the differential signal is further differentiated, high frequency noise increases. As a result, the binary signal includes greater jitter than that obtained by the above-described level slicing technique.
In view of the above, it is an object of the present invention to provide an AGC system and an information reproduction apparatus which can control the amplitude level of a reproduction signal such that it is always maintained at a predetermined level regardless of MTF characteristics of an optical system, and which can also generate a reproduction signal well suited both for binarization of a mark-edge-recording signal and for binarization of a mark-position-recording signal.